Preheat the oven to 160°C/140°C fan-forced. Line a large oven tray with non-stick baking paper; mark a 22cm circle on paper. Grease and dust with cornflour, shake away excess.

Beat eggwhites in a large bowl with an electric mixer until soft peaks form, gradually add sugar, beat until dissolved between additions.
When sugar is dissolved, gently fold in sifted cocoa, vinegar and chopped chocolate.

Place pavlova in oven and immediately reduce the temperature to 120°C/100°C fan-forced. Bake for about 1¼ to 1½ hours or until dry to touch. Turn oven off and cool pavlova in the oven with the door ajar.

Just before serving, place pavlova onto serving plate. Place mascarpone in a bowl, gently whisk in enough milk to form a soft consistency. (Mascarpone varies in texture so add gradually and only if needed.) Spoon mascarpone mixture on top of pavlova and sprinkle with berries. Decorate with extra chocolate, if desired.
Not suitable to freeze or microwave.